Rhodium compounds carbonyl clusters

There are many related compounds, including rhodium carbonyl cluster anions, which are present in the solutions cataly2ing ethylene glycol formation and which may be the catalyticaHy active species or in equiUbrium with them (38). 

Ni and Co or of oxophilic metals, for example. Re, is still poorly studied the surface-mediated synthesis of bimetallic carbonyl clusters is limited to a few examples the surface-mediated synthesis of metal compounds without carbonyl ligands has just begun with the silica-mediated synthesis of [RhH2(PMe3)4] by treatment of bis (allyl) rhodium with PMe3 followed by H2 [121] the silica-mediated synthesis of tantalum clusters has been investigated recently but the products were not extracted from the surface-for example, treatment of silica physisorbed Ta(CH2Ph)5 in H2 at 523 K for 20 h led to tri-tantalum clusters, as shown by EXAFS spectroscopy [122]. 

Rhodium carbonyl complexes catalyze the silane-initiated cascade cyclization of 1,6,11-triynes to form fused aromatic tricyclic compounds. For example, reaction of 83 [X = G(G02Et)2] with methyldiphenylsilane catalyzed by the tetrarhodium carbonyl cluster Rh4(GO)i2 in toluene at room temperature gave an 88 12 mixture of the silylated and unsilylated fused tricycles 84a and 84b [X = G(G02Et)2] in 85% combined yield (Equation (55)). The ratio of silylated to unsilylated tricyclic product formed in the reaction of 1,6,11-triynes was dependent on the nature of the substrate (Equation (55)). For example, Rh4(GO)i2-catalyzed reaction of diaminotriyne 83 (X = NBn) with methyldiphenylsilane gave unsilylated tricycle 84b (X = NBn) in 92% yield as the exclusive product (Equation (55)). 

For hydroformylation over cobalt and rhodium zeolites the active species have not been defined. However, in the case of RhNaY the in situ formation of a rhodium carbonyl cluster has been identified (226) by infrared spectroscopy. Interestingly, this cluster appears to be different from known compounds such as Rh4(CO)12 and Rh6(CO)16. This does suggest that alternative carbonyl clusters may possibly be formed in zeolites due to the spatial restrictions of the intracrystalline cavities. The mechanism of hydroformylation in these zeolites is probably similar to that known for homogeneous catalysis. 

Tri-/i-carbonyl-nonacarbonyltetrarhodium is the starting material for the synthesis of a large number of rhodium carbonyl cluster compounds.10... 

The selective production of methanol and of ethanol by carbon monoxide hydrogenation involving pyrolysed rhodium carbonyl clusters supported on basic or amphoteric oxides, respectively, has been discussed. The nature of the support clearly plays the major role in influencing the ratio of oxygenated products to hydrocarbon products, whereas the nuclearity and charge of the starting rhodium cluster compound are of minor importance. Ichikawa has now extended this work to a study of (CO 4- Hj) reactions in the presence of alkenes and to reactions over catalysts derived from platinum and iridium clusters. Rhodium, bimetallic Rh-Co, and cobalt carbonyl clusters supported on zinc oxide and other basic oxides are active catalysts for the hydro-formylation of ethene and propene at one atm and 90-180°C. Various rhodium carbonyl cluster precursors have been used catalytic activities at about 160vary in the order Rh4(CO)i2 > Rh6(CO)ig > [Rh7(CO)i6] >... 

The controlled synthesis of Ti-d8 early-late heteropolynuclear diolefin and carbonyl clusters has been reported. The synthetic approach was based on deprotonation reactions involving Gp2Ti(SH)2 and appropriate rhodium and iridium diolefin and carbonyl compounds. The catalytic activity of some representative Ti-Rh compounds toward alkene hydroformylation has been explored.1778... 

Several rhodium and ruthenium clusters are reported to catalyze the reduction of carbonyl compounds [Table III (70,108- 112c)]. The hydrogenation of aldehydes and ketones yields the corresponding primary and secondary alcohols... 

Rhodium carbonyl clusters. 13, 288 15. 3j Hydrogenation. Rh6(CO)i6 catalyzes compounds with CO and H2O at atmospheric Homologations. Hydroformylation of ated by Rh4(CO)i2. Homologous carboxylic ai obtained from allyl phosphates. ... 

Wider possibilities are also available with cluster-t)T3e MCMs. They are molecular compounds that include a framework of metal atoms, which are separated by short distances (not more than 3.5 A) permitting direct metal-metal interaction. Such a framework is enclosed within a set of polymerizable ligand groups. The first report in this regard relates to MCMs based on Co2(CO)g or Fc2(CO)9 and the methyl ester of j7-vinyldithiobenzoic acid as well as Wcyclohexyl-4-vinylthiobenzamide [57]. MCMs based on a carbonyl cluster of osmium 89, ruthenium [58] and rhodium [59] with 4-VPy 90 and allyldiphenylphosphine 91 are stable compounds (see Experiments 4-9 and 4-10, Section 4.6) [59,60]. 

A number of binuclear phosphine-ligand bridged complexes have been shown to function as catalysts or catalyst precursors. Here we review these cases briefly. More detailed coverage of hydrogenation and hydroformyla-tion using rhodium catalysts will appear in the next chapter. In all cases of catalytic activity shown by binuclear complexes, there is a serious question about the true identity of the catalytically active species. As with catalysis begun by metal carbonyl clusters, the possibility exists that a small amount of highly active mononuclear compound is the true catalyst. 

The compound Rh4(CO)i2 has a large number of applications. It is used in catalysis directly or as a catalyst precursor, and it is the starting material both for substitution reactions and for the synthesis of other rhodium carbonyl clusters. In particular it is easily converted into Rh (CO)i4, thus providing a method for the synthesis of this carbonyl in a highly pure state. ... 

Cluster Compounds of Co, Rh, and Ir. In addition to the above-mentioned neutral cluster compounds, there is a large number of anionic carbonyl clusters and metal carbonyl carbides. Carbonyl carbides are formed when the interstice inside the metal cluster is sufficiently large to accommodate the carbon atom. Carbonyl carbides possessing at least four metal atoms are known. The most thoroughly investigated carbides are those of rhodium because they are very stable and resist air oxidation. Carbonyl clusters of group 9 elements containing even more than 20 metal atoms are now known [M6(CO)i5] - (M = Co, Rh, Ir), lM CO)uT. [M6(CO)i5C]"-(M = Co,Rh), [Co8(CO),sC] -, [Rh,(CO)i,] -, [Rh8(CO)i,C], [Ir8(CO)22]"-,... 

The cobalt carbonyls are prepared by the disproportionation reaction of [Co2(CO)g] in the presence of Lewis bases or by the reduction of cluster cobalt carbonyls with the alkali metals. The iridium compounds are obtained during reduction of [Ir4(CO)i2] with sodium in ether solution. The rhodium carbonyls are usually synthesized by reduction of [Rh2Cl2(CO)4] or [RhClg] " with carbon monoxide in basic medium or by nucleophilic attack of bases on the carbonyl group of carbonyl clusters (see preparation of [M4(CO)i2] and [M6(CO)i6]). 

Other organic processes facilitated by metal carbonyl clusters include a palladium carbonyl catalysed Diels-Alder reaction the selective reduction of aromatic nitro compounds using rhodium and ruthenium phosphine-carbonyls aza- and oxa-carbonylations of allyl phosphates by rhodium carbonyls Michael reactions of alkoxy-alkenones using iron... 

Retaining the theme of metal carbonyl clusters, capping considerations in transition-metal clusters have been discussed with reference to [Sb2Co4(CX))] g( A-CX))], and [Bi2Co4(CO)jQ( i-CO)]" 28. An infrared spectroscopic study of the formation of carbonyl rhodium clusters on a rhodium electrode produced by oxidation reduction cycles in acidic solution 2 has also been published. Electrochemistry with ruthenium carbonyls >21 osmium carbonyls 2 jg also reported. Muon spin rotation in a metal-cluster carbonyl compound has been communicated and, lastly, a proton spin-lattice NMR relaxation study of hydride carbonyl clusters has been reported. This provides a method for determining distances involving hydrido ligands... 

The use of the cobalt triad carbonyls as catalysts continues to provide many papers for this report. Publications cover the silylformylation of 1-Hexyne catalyzed by diodium-cobalt carbonyl clusters the formation of hydroxycarbene cobalt carbonyl derivatives, the use of rhodium cluster carbonyls in the water-gas shift reaction Rh4(CO) 2> and Co3Rh(CO)] 2 catalysts for the hydrosilation of isoprene, cyclohexanone and cyclohexenone catalytic reduction of NO by CO and the carbonylation of unsaturated compounds The chemistry of iridium carbonyl cluster complexes has been extended by making use of capping reactions with HgCl2and Au(PPh3)Q... 

The structures of Rh4(CO)i2 and Rh6(CO)j6 are shown in Figures 18.4 and 18.5, respectively [40,41]. These compounds form cluster structures in which rhodium atoms bond each other. These consist of Rh-Rh bond, Rh-CO bond and a bridged carbonyl bond in which the carbon of carbonyl group bonds to two or three rhodium atoms. As Rh6(CO)i6 has more Rh-Rh bonds compared with Rli4(CO)i2, it is more stable and has less solubility. 

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